The present invention relates to battery chargers and in particular to a battery charging circuit in which the temperature of a battery is sensed to control the charging of the battery through parallel charging paths.
Rechargeable sealed batteries such as multiple cell nickel cadmium batteries are commonly used in small electrical applicances and devices as the main power supply. Such devices are typically run from the battery supply without connection to an external source of power until the need for recharging arises. Thereafter, the device is typically coupled to a source of charging current such as a rectifier powered by normal AC current available in the home until the battery is recharged.
The recharging of such batteries can proceed at a relatively slow pace without too much danger of damaging the battery. However, for many uses it is desirable to recharge the battery as rapidly as possible so that the battery powered device is available for use without for use without too long a wait.
Fast charging systems for nickel cadmium batteries typically employ a temperature sensitive element to control the charging time and/or rate. A well known characteristic of nickel cadmium batteries is that the battery temperature exhibits a sharp rise near the point at which the battery goes into overcharge when the battery is charged at a fast rate. This temperature rise, apparently caused by the onset of oxygen generation and its recombination reaction in the cell, can be monitored by sensing the cell case temperature and has been used in various ways to change the charging rate or completely disconnect the battery from the source of charging current. For example, in one such system described in U.S. Pat. No. 3,928,792 issued to Mullersman et al, a manually set, temperature actuated switch normally coupled to the battery terminates fast charging of the battery by disconnecting the battery from the charging current source. It is further suggested in the Mullersman et al patent that the battery may be placed on a trickle charge via a bypass resistor in parallel with the temperature actuated switch after the switch has been actuated. In order to reset the temperature actuated switch so that the battery can be charged after use, the trigger mechanism for energizing the device from the battery is linked to the temperature actuated switch to manually reset the switch when the appliance is used.
In another system illustrated in U.S. Pat. No. 3,601,679 issued to Braun et al, the battery temperature is sensed during charging, and a relay coil actuated by the temperature sensing element opens a set of relay contacts to disconnect the battery charger from the battery when the battery temperature exceeds a predetermined value. The relay contact is arranged such that it is held in the open position even after the battery temperature returns to a lower value so that the battery remains disconnected from the charger once the predetermined temperature has been exceeded. Various ways are suggested by Braun et al for returning the relay contact to its closed position for subsequent charging operations.
While the foregoing and other known systems provide for fast charging of rechargeable cells, it has been found desirable to provide a relatively simple, highly reliable temperature controlled battery charged circuit wherein a minimum of inexpensive circuit elements is employed to automatically terminate the rapid charge above a predetermined battery temperature and to continue a trickel charge upon termination of the rapid charge, with the automatic reset of the circuit for subsequent charging upon interruption of all charging current to the battery.
It is accordingly an object of the present invention to provide a novel battery charging system having the aforesaid desirable characteristics.
It is another object of the present invention to provide a highly reliable, extremely simple and inexpensive battery charger employing an automatically resetable (i.e., normally closed) temperature responsive switch for changing the charging rate of a battery from a fast to a trickle rate upon the battery temperature exceeding a predetermined value wherein the current path for the fast charge remains open as long as a trickle charge is being supplied to the battery.
It is a more specific object of the present invention to provide a normal battery charger in which two parallel connected current paths, one containing a temperature responsive switch and a relay contact, and the other containing the coil for actuating the relay contact, supply fast charging currentand trickle charging current to a battery in response to battery temperature wherein supply of current through the two paths is essentially mutually exclusive and the circuit is automatically reset for subsequent charging when all charging current in interrupted by disconnection of the battery from the charger or the like.
These and other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.